Extraction process



Jan. 29, 1957 G. H. DALE Erm. 2,779,709

EXTRACTION PROCESS 2 Sheets-Sheet il Filed Dec. 3l, 1952 Jan. 29, 1957 G. H. DALE ETAL 2,779,709

EXTRACTION PROCESS 2 Sheets-Sheet 2 Filed Dec. 3l, 1952 S j M d M H M M u I/ mw w M m m 7 Mw k kmbQOQQ m nkm 4 l. mh

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INVENTORJ` alf/Me @d BY ashezzm UWM nited States EXTRACTIGN PROCESS Application Decemberl 31, 1952, SeriaNo. 329,620 1S Claims. (Cl. 19o-14.19)

This invention relates to the solvent extraction of hydrocarbon mixtures. In one aspect -this invention relates to a process employing sulfur dioxide in conjunction with a parafinic or naphthenic stripping oil in the separation of aromatic hydrocarbon Vfrom petroleum fractions. In still another aspect this invention relates to a process wherein sulfur dioxide is removed from an extract phase formed during sulfur dioxide solvent extraction of a petroleum fraction, in an amount to cause residual extract to closely approach but not reach miscibility with a `paratlinic stripping oil when contacted therewith in a subsequent extraction step, whereby maximum rate of production of a final aromatic extract or concentrate at a given purity level is achieved.

As disclosed in your copending application Serial No. 262,754, tiled December 2'1, 1951, now Patent No. 2,724,682 in the sulfur dioxide solvent extraction of aromatic hydrocarbons from petroleum fractions, the solubility relationship between sulfur dioxide and aromatic hydrocarbons is altered by the presence of a paraiiinic or naphthenic stripping oil. The latter, utilized as described "hereinafter, operates substantially `as an extraction solvent and as such extracts less aromatic constituents from the petroleum lfraction treated to provide extract freed from parainic or naphthenic hydrocarbons to an extent markedly greater than that achieved by utilization of liquid sulfur dioxide alone as a selective solvent.

in one form of utilization of sulfur dioxide in conjunction with a stripping oil vin Vthe extraction of aromatics from a petroleum fraction, the extraction process is carried out in two steps." In the first step the charge stock is contacted with liquid sulfur dioxide in the presence or absence of a paratlinic or naphthenic stripping oil, as desired. In a second step extract phase `from the iirst extraction step is contacted as a separate phase with a suitable stripping oil under which conditions a i'lnal extract phase is obtained, -Which when freed of solvent, constitutes an aromatic extract or concentrate of high purity, i. e., it `has an aromatic content greater than `that obtained when employing sulfur dioxide alone as an extraction solvent.

in accordance with our invention we provide a solvent extraction process comprising solvent extracting aromatico from a hydrocarbon fraction with sulfur dioxide alone as the selective solvent or in conjunction with a parain or naphthenic stripping oil; removing sulfur dioxide from resulting "primary extract phase, in an amount to provide residual primary extract phase that approaches but does not reach .complete miscihi'lity with a paranic or naphthenic stripping oil `to be contacted therewith; and lcontacting resulting residual primary extract phase with the stripping oil "under solvent extraction conditions, thereby forming inal aromatic extract or concentrate at `a maximum rate of production atany given level of nal extract purity.

Our invention is based on Lour discovery that, when solvent extracting an aromatic-containing hydrocarbon arent ICC mixture, with liquid sulfur dioxide alone or in conjunction with a stripping oil, and then solvent extracting resulting primary extract phase with a stripping oil, the purity, i. e. aromatics content, of the iinal aromatic extract passes through a minimum as the proportion of hydrocarbon in the solvent treated primary extract phase is increased from that of the total primary extract phase 'to a concentration causing the residual primary extract phase to closely approach but not reach complete miscibility with the stripping oil. We have found as a result of our discovery that inal extract at any given purity level can be produced when the concentration of hydrocarbon in the solvent `treated residual primary extract phase is at either one of two possible values; and further, that the rate of production of the iinal aromatic extract is markedly higher `when the concentration of hydrocarbon the `solvent treated residual primary extract phase is -the higher of its two possible values.

Accordingly our invention provides for the removal of sulfur dioxide from total primary extract phase in an amount so that the concentration of hydrocarbon in the resulting residual primary extract is of a va'lue suiiiciently high that when the lresidual primary extract phase is solvent extracted with stripping oil, final extract product will be produced at the higher of `the two possible production rates, thereby providing for a maximum rate of production of nal extract product at any given level of purity.

Qur invent-ion is graphically illustrated in Figure 1 of drawings with reference to a toluene-normal heptane mixture. Figures la and 1b illustrate that in the solvent extraction with a parailinic stripping oil of an extract containing normal heptane Vin a weight ratio `to toluene of about 1:1 and recovered Afrom liquid sulfur dioxide extraction of a toluene-normal heptane mixture at a given stripping oil rate, the purity of the tinal extract, i. e., product of Vstrirnging oil extraction, passes through a minimum as the concentration of hydrocarbon in the primary extract phase, increases from that of the `initial primary `extract to a value just `short of causing the primary extract to` be completely miscible with stripping oil 'in the stripping `oil solvent extraction step. `)Figures la and lb further illustrate our discovery that final extract product at any given `purity level can be produced at two possible rates, each dependent upon the concentration of hydrocarbon in the primary extract introduced into the stripping oil extraction system. nFigure l shows total toluene production (curves i) and the `toluene purity (curves 2) for varying concentrations of hydrocarbon in the primary extract material that is solvent extracted `with stripping oil. Figure la is based on a stripping oil rate or" 4.8 pounds per hour and a sulfur dioxide rate of .43.2 pounds per hour, and Figure lb on a stripping oil `rate of 9.6 pounds per hour and a sulfur dioxide rate of 43.2 pounds per hour, the `temperature in each case being 30 i?. Thus, Figure lb shows that 95 percent toluene can be produced at a rate of 1.5 pounds per hour if the hydrocarbon content of the primary extract phase treated is 8.2. `weight percent, but that, if :the said hydrocarbon content is increased to 14.1 weight percenhrhe 95 percent toiuene is produced at a rate or 2.5 pounds per hour. Accordingly, when ,the primary extract has the higher hydrocarbon coutent, rate of production -`of the nal 95 `percent purity .extract product `is more than 65 percent greater than the rate of production obtained when the `'said hydrocarbon content is the lower of ,the tivo values. Similarly, Figure la shows n at 8,() percent toluene can be produced at a rate of about 2 pounds per hour when the hydrocarbon content of the extract phase `treated is 9 weight percent hydrocarbon but that, when the hydrocarbon content of the primary `extract treated is 13 weight percent hydrocarbon, the percent 'toluene is produced at a `rate of 2.7

pounds per hour, which is an increase in rate of production of 80 percent toluene of about 35 percent.

Sulfur dioxide removal from the primary extract phase, as illustrated in Figure l, `can be regulated to eifect varying degrees of approach tomiscibility of residual primary extract with stripping oil in the final extraction step, production at the maximum of two possible rates being obtained at any given purity level so long as the proportion of hydrocarbon in residual extract treated is not less than that at which the minimum purity of iinal extract is obtained. Accordingly, when referring herein to the step of increasing the proportion of hydrocarbon in primary extract to a value causing resulting residual extract to approach miscibility with stripping oil in the subsequent extraction step, it is meant to remove at least a sufcient amount of sulfur dioxide to provide a concentration of hydrocarbon in the residual extract to be .further treated, sufficiently high that the purity of the final extract product is greater than its minimum possible value. Preferably we form residual primary extract by removal of the maximum permissible proportion of sulfur dioxide so as to utilize maximum permissible hydrocarbon concentrations to achieve not only a maximum rate of production of final extract product at a given purity level but also to achieve maximum product purity.

Our invention is further illustrated with reference to Figure 2 of the drawings, which is a diagrammatic flow sheet setting forth a preferred embodiment of our process. Referring to Figure 2, a hydrocarbon feed mixture containing aromatic hydrocarbons is introduced through line via pump 11 and line 12 into a lower portion of ex- 'tractor 13 and conntercurrently contacted therein with liquid sulfur dioxide from surge tank 36 introduced into an upper portion of extractor 13 through line 14. Also introduced into extractor 13 via line 12 is a paraifinic or naphthenic stripping oil from recycle line 16, preferablv boiling in a range above that of the hydrocarbon feed introduced into extractor 13 from line 10. Extractor 13 is maintained under conditions for effecting solvent extraction of aromatic hydrocarbons from the hydrocarbon feed mixture charged, and is generally operated under a pressure sufficiently high to maintain all materials therein in liquid phase and at a temperature within the range of about 30 to 70 F., more generally from about 30-40 F. Total raffinate `is withdrawn from an upper portion of extractor 13 via line 18 and passed to separation chamber 19. Sulfur dioxide is separated from rafnate in chamber 19, e. g., by heat from steam coil 21, Sulfur dioxide-free raffinate is withdrawn from chamber 19 and passed via line 22 into rainate fractionator 23. Stripping oil in fractionator 23 is withdrawn through line 24 and passed via line 26 into stripping oil tank 27. Raffinate product is withdrawn from fractionator 23 via line 28.

Total primary extract phase is withdrawn from a lower portion of extractor 13 via line 29 and passed through pressure control valve 31 into sulfur dioxide flash chamber 32, wherein a controlled portion of the sulfur dioxide component is flashed therefrom. Sulfur dioxide removed from primary extract in flash chamber 32 is withdrawn via line 33 and passed via pump 35 through line 40 into sulfur dioxide surge tank 36. Sulfur dioxide is removed from total primary extract in chamber 32 in an amount so as to provide residual primary extract phase having a hydrocarbon content sutliciently high that, when the resulting residual extract phase is solvent extracted with a stripping oil in a subsequent step (extractor 34), the said residual extract will closely approach but will not reach complete miscibility with the stripping oil. In other words, it is the purpose of the operation of chamber 32 to increase the hydrocarbon content of the primary extract phase from extractor 13 to a point just short of causing the resulting residual phase to reach complete miscibility with the stripping oil in extractor 34. Due to the n arroyos j Y' 4 removal of a portion of the sulfur dioxide from the primary extract a small amount of hydrocarbon may separate as layer 37 therefrom, which is Withdrawn via line 3S and pump 39 and recycled via line 12 to extractor 13. Residual extract phase in chamber 32 is Withdrawn via line 41 and pump 45 and passed into an upper portion of extractor 34 and contacted countercurrently therein with parafiinic stripping oil introduced into a lower portion of extractor 34 from tank 27 via line 42, the said stripping oil having a boiling range above that of feed in line 10, as described hereinabove. Extractor 34 is maintained under conditions for eifecting solvent extraction of lessaromatic components from residual primary extract therein, thus providing for a iinal extract phase of increased aromatics content, which is Withdrawn from a lower portion of extractor 34 through line 43 and passed to separation chamber 44. Solvent extraction in extractor 34 is operated in liquid phase at a temperature generally in the range of 30-50 F., preferably about 3040 F., although temperatures outside this range can be employed if desired, particularly higher temperatures. The ratio of stripping oil to residual primary extract phase introduced into extractor 34 can be varied over a Wide range as desired, as for example a weight ratio within the range of 0.0521 to 0.50z1, although higher ratios outside such a range can be employed if desired.

Sulfur dioxide is separated from extract product in separator 44, e. g., by heat supplied by a steam coil 46. Sulfur dioxide is Withdrawn from separator 44 via line 47 and passed through line 40 into sulfur dioxide surge tank 36, Residual extract in chamber 44 is withdrawn therefrom and passed through line 48 into extract fractionator 49, wherein stripping oil is separated from the residual extract, the latter being withdrawn from fractionator 49 through line 51. Stripping oil separated in fractionator 49 is withdrawn through line 52 and passed via line 26 into stripping oil tank 27.

The stripping oil useful in our process may be a straight run hydrocarbon oil, as for example a low viscosity lubricating oil fraction, or it may be a kerosene or gas-oil. The oil may contain both parafnic and naphthenic hydrocarbons or may be completely parainic or naphthenic -in nature. Naphthenic hydrocarbons are cycloparafiinic hydrocarbons, and accordingly the generic term parainic hydrocarbon as 'applied to the stripping oil and used in this specification and claims is intended to include straight chain paraffinic, isopara'inic and/ or naphthenic type hydrocarbon oils.

The stripping oil also should possess an initial boiling point somewhat higher than the end boiling point of any of the hydrocarbons being treated, since if there is an overlapping of boiling points, feed stock hydrocarbons may accumulate in the stripping oil or stripping oil hydrocarbons may be removed from the system in one of the hydrocarbon products. The distillation end point of the stripping oil is not particularly critical, but when this temperature is too high, freezing points of high boiling constituents may disadvantageously limit desirably low operating temperatures. A particularly suitable parafiinic oil is a vacuum distillate oil having a viscosity somewhat less than that of an SAE 10 motor oil, and having an initial boiling point of about 600 F. We find that an oil of this type is very satisfactory as a stripping oil in carrying out our invention.

Our invention is preferably applied to the solvent extraction of gasoline range boiling hydrocarbons although applicable to heavier hydrocarbons such as those boiling in the kerosene range. Exemplary of hydrocarbon fractions to which our invention is particularly applicable are catalytically reformed gasolines and thermally cracked naphthas.

Although the hydrocarbon concentration in the residual primary extract to be solvent extracted with stripping oil in extractor 34 is largely dependent upon the particular aromatic feed stock introduced into the syssidual primary extract is somewhat smaller.

Our invention is further illustratedby way of the'following data characteristic of solvent extraction'of a toluene-normal heptane mixture` charged to' ourprocess of Figure 2. The data, descriptive ofstream compositions, are associated with designated`.=.`len'1entsA lfFigure 2 and are expressed" in terms of poundsper hour.

components 1o "431su "14 42 `29V s3 41" 12l 3s Tolueneth.. 5.4 4.8 0.6 N-Heptane.; 20.0 0.2 19.8`

Sulfur Dioxide 78.0 20.0 1950.---. 182.8 `97.0 85. Stripping Oil- 1.8 17.2 19.0 3.0 1

Variation and modification are possible within the scope of the foregoing disclosure and the appended claims to this invention, the essence of which is that in the sulfur dioxide extraction of aromatic hydrocarbons from a petroleum fraction, wherein the feed fraction is contacted in a iirst stage with liquid sulfur dioxide in the presence if desired, of a stripping oil, and resulting extract is then passed to a second stage and solvent extracted therein with a stripping oil, a controlled portion of sulfur dioxide is removed from the first stage total extract prior to its introduction into the second stage, in an amount such that residual extract approaches but does not reach complete miscibility with the second stage stripping oil, whereby a maximum rate of production of aromatics is obtained at any given aromatics purity level.

We claim:

l. In a sulfur dioxide solvent extraction process in which a primary extract phase formed is contacted with a paraliinic stripping oil which is substantially free of sulfur dioxide to produce a nal extract product, the irnprovement which comprises removing sulfur dioxide from said primary extract phase in an amount to form a residual primary extract phase which is less than completely miscible with said stripping oil, and then contacting with said stripping oil said residualV primary extract phase, in the condition in which it is obtained and while maintaining it in its said less than complete miscibilitycondition with said stripping oil, stripping said primary extract phase with said stripping-oil While it is maintained in its said condition, and separating a phase containing said stripping oil and a stripped extract phase each fromV the other.

2. in a process for therecovery of aromatics from a petroleum fraction containing the same and comprising the steps of solvent extracting such a fraction with liquid sulfur dioxide solvent, solvent extracting the resulting primary extract phase with a paraiiinic stripping .oil which is substantially free of sulfur dioxide, and recovering` the resulting aromatic extract as a product of the process, the improvement which comprises increasing the proportion of aromatic hydrocarbons in the said primary extract phase prior to said stripping oil solvent extraction step by removal of sulfur dioxide to forman extract phase which is less than completely miscible with the said stripping oil, then contacting said extract phase with said stripping oil while maintaining it in said less than completely miscible condition with said stripping oil, and

separatingr a phase containing 'said stripping oil and a stripped extract phase.` each .from the other.`

3*. In` a sulfur dioxide solvent extraction process in which the primaryextract phase formed is contacted with a parainic stripping oil which is substantially free ot sulfur dioxide to produce a final extract product, the improvement whichy comprises removing a determined quantity of sulfur dioxide from said extract phase, determining said quantity removed by removing SO2 from said extract phase until the purity of the aromatic oil recovered, when all SO2 Vhas been separated from said phase, following its later treatment with a parainic stripping oil, begins to decrease and then reaches a minimum, continuing `the removal ofsulfur dioxide from said phase until said purityof said oil recovered has increased appreciably beyond said minimum,` and then stripping the residual primary extract phaser thus obtained with said parafiinic stripping oil while maintaining said residual primary extract phase in the condition which it has reached when said determined quantity of SO2 has been removed.

4. In a sulfur dioxide solvent extraction process in whicli'the primar-yfextracr phase formedis contacted with a paraiiinic stripping oil which isfisubstantially free of sulfur dioxide to produce a inal extract product, the `improvement which comprises removing a determined quantity of sulfur dioxide from said extract phase, determining said quantity removed by removing SO2 from said extract phase, until the purity of the aromatic oil recovered, when all SO2 has been separated from said phase, following its later treatment with a paratinic stripping oil, begins to decrease and then reaches a minimum, continuing the removal of sulfur dioxide from said phase until said purity of said o-il recovered has increased substantially beyond said minimum, and then stripping the residual primary extract phase thus obtained with said parafnic stripping oil while maintaining said residual primary extract phase in the condition which it has reached when said determined quantity of SO2 has been removed.

5. in a process for the recovery of aromatics from a petroleum fraction containing the same, in which said petroleum fraction is extracted with a liquid sulfur dioxide solvent, the steps comprising extracting the resulting extract phase with a paratlinic stripping oil which is substantially free of sulfur dioxide and also comprising the steps as follows: removing a determined quantity of sulfur dioxide from said extract phase, determining the quantity of SO2 to be removed by removing SO2 and plotting rate of production and purity of a final aromatic extract, obtained after contact with said stripping oil, for each residual primary extract phase concentration, thus obtaining a curve which passes through a minimum value as the concentration of hydrocarbon in the extract phase is increased and then contacting an extract phase from Vwhich said determined quantity of SO2 has been removed and which is represented by a point on said curve above said minimum value with said stripping oil, thus producing a final aromatic extract product at one of two possible production rates at the purity level at which said extract phase, from which sulfur dioxide has been removed has been selected.

6. ln a process for the recovery of aromatics from a petroleum fraction containing the same, in which said petroleum fraction is extracted with a liquid sulfur dioxide solvent, the steps comprising extracting Ithe resulting extract phase with a parafiinic stripipng oil which is substantially Vfree of sulfur dioxide and also comprising the steps as follows: removing a determined quantity of sulfur dioxide from said extract phase, determining the quantity of sulfur dioxideto be removed by removing sulfur dioxide and plotting rate of production and purity of a final aromatic extract, obtained after contact with said stripping oil, for each residual primary extract phase concentration, thus obtaining a curve which passes through a minimum value as the concentration of hydrocarbon in the extract phase is increased and then contacting an extract phase from which said determined quantity of SO2 has been removed and which is represented by a point on said curve above said minimum value on the positive slope portion of said curve, with said stripping oil, thus producing a final aromatic extract product at the maximum rate possible at said purity level.

7.1n la sulfur dioxide solvent extraction process in which the primary extract phase formed is contacted with a paraiiinic stripping oil which is substantially free of sulfur dioxide to produce a nal extract product, the improvement which comprises determining the quantity of sulfur dioxide to be removed from the extract phase to obtain a resulting extract phase which is less than completely miscible with `the said stripping oil, removing said quantity of sulfur dioxide from said primary extract phase7 and stripping said primary extract phase while maintained in its said less than complete Vmiscibility condition with only the said stripping oil. v

8. A process of claim 2 wherein said petroleum fraction contains normal heptane and toluene.

9. A process of claim 2 wherein said petroleum fraction boils in the gasoline range.

10. A process of claim 2 wherein said petroleum frac tion boils not-higher than the kerosene boiling range.

11. A process of claim 2 wherein the rst said solvent extraction is conducted atea temperature within the range of 30 to 70 F.

12. A process of claim 2 wherein the last said solvent extraction is effected at a temperature Within the range of 30 to 70 F. p

13. A process of claim 2 wherein said stripping oil has a boiling range above about 600 F.

14. A process of vclaim 2 wherein the initial boiling point of said stripping oil is above the boiling range of said petroleum fraction. f

15. A process of claim 2 wherein the maximum conycentration of aroma-tic hydrocarbon in said residual extract does not exceed 20 weight percent.

References Cited in the le of this patent UNITED STATES PATENTS 

1. IN A SULFUR DIOXIDE SOLVENT EXTRACTION PROCESS IN WHICH A PRIMARY EXTRACT PHASE FORMED IS CONTACTED WITH A PARAFFINIC STRIPPING OIL WHICH IS SUBSTANTIALLY FREE OF SULFUR DIOXIDE TO PRODUCE A FINAL EXTRACT PRODUCT, THE IMPROVEMENT WHICH COMPRISES REMOVING SULFUR DIOXIDE FROM SAID PRIMARY EXTRACT PHASE IN AN AMOUNT TO FORM A RESIDUAL PRIMARY EXTRACT PHASE WHICH IS LES THAN COMPLETELY MISCIBLE WITH SAID STRIPPING OIL, AND THEN COMTACTING WITH SAID STRIPPING OIL SAID RESIDUAL PRIMARY EXTRACT PHASE, IN THE CONDITION IN WHICH IT IS OBTAINED AND WHILE MAINTAINING IT IN ITS SAID LESS THAN COMPLETE MISCIBILITY CONDITION WITH SAID STRIPPING OIL, STRIPPING SAID PRIMARY EXTRACT PHASE WITH SAID STRIPPING OIL WHILE IT IS MAINTAININED IN ITS SAID CONDITION, AND SEPARATING A PHASE CONTAINING SAID STRIPPING OIL AND A STRIPPED EXTRACT PHASE EACH FROM THE OTHER. 